The XMM-Newton space telescope has been really into dark and heavy stuff lately. No, the x-ray observatory didn't find its parents' old Black Sabbath albums — it found the most massive galactic cluster on record, a conglomerate of galaxies weighing more than 1,000 times as much as the Milky Way. The cluster's extreme distance is a puzzling paradox, or it might help prove that the universe is full of mysterious dark energy.Last week, we told you about two galactic clusters colliding, providing visual (and x-ray) evidence of the existence of dark matter. Dark energy is even more elusive, but astronomers who were looking at an entirely different cosmic object may have stumbled across a major clue. The large blue glow in the image (taken with the Large Binocular Telescope at the Mt. Graham International Observatory to confirm XMM-Newton's discovery) is a galactic cluster with the catchy name 2XMM J083026+524133. It's heavier than any other cluster, and it's farther from Earth than any other as well. In fact, it's twice as far as the next farthest. Here's the mystery — really distant clusters should be very young, because it takes their light so long to reach us. Young clusters shouldn't be big, because it takes time for all those galaxies to pull together. A huge, distant cluster is seemingly impossible. That's where dark energy comes in. Dark energy, to put it into very simple terms, is a sort of anti-gravity force that actually pushes space apart and speeds the expansion of the universe. If it exists, that is. It is one possible explanation for 2XMM J083026+524133's distance. It might be an old cluster that appears more distant than it should be due to dark energy fueled expansion of space. What is the source of all the dark energy in the universe? Tony Iommi. Image by: ESA.Cosmic Heavyweight. [Science News]